1. Field of the Invention
The present invention relates to printed circuit boards and their manufacturing methods, more particularly, to printed circuit boards having an excellent heat dissipation performance and their manufacturing methods.
2. Description of the Prior Art
Recently, there is a growing need for printed circuit boards having a high heat dissipation performance as electronic components are being mounted in high-density. A metal core substrate has been known as a printed circuit board having a high heat dissipation performance and has already been used in practice. The metal core substrate, in which a metal of high thermal conductivity such as aluminum and copper is used as its core material, can disperse heat from heat generating components over the substrate, which prevents the heat generating components from rising in temperature. Among others, aluminum, which has a thermal conductivity of 236 W/mK and a specific gravity of 2.7 g/cm3, is a relatively lightweight material; hence used generally for core materials (for example, refer to Japanese Laid-Open Patent No. 1996-97556, para. [0006], FIG. 8).
Nowadays, carbon fiber reinforced plastic (hereinafter abbreviated to “CFRP”) is also considered as another core material that has a lower thermal expansion, higher strength, and lighter weight than those of aluminum (for example, refer to Japanese Laid-Open Patent No. 1999-40902, para. [0008]-[0015], FIG. 9).
CFRP is a composite material that is composed of carbon fiber and resin, and its thermal expansivity, thermal conductivity, strength, and specific gravity are tunable by varying its carbon fiber content and structure such as cross structure or unidirectional structure (for example, refer to Japanese Laid-Open Patent No. 2003-273482, para. [0034]-[0049], FIG. 16). Generally, carbon fiber having thermal conductivity of 140 to 800 W/mK is available on the market. By being laminated with prepregs containing an unidirectional structure carbon fiber of a thermal conductivity of 620 W/mK with their fiber orientations being in order of 0°, 90°, 90°, and 0°, a CFRP plate can be obtained that has a thermal expansivity of approximately 0 ppm/° C. (in plane), a thermal conductivity of 217 W/mK, an elastic modulus of 290 GPa, and a specific gravity of 1.6 g/cm3. The CFRP plate, while keeping heat dissipation performance comparable with that of aluminum, exhibits as a core material a lower thermal expansivity, higher strength, and lighter weight than those of aluminum. Accordingly, by making a core substrate with the CFRP, a substrate can be obtained that has a higher performance, in particular, a more excellent mounting reliability than those of aluminum, without occurrence of cracks at soldered connection portions even in a case of large ceramic parts being mounted.
Since any core materials mentioned above, however, are electrically conductive, a core layer and throughholes formed for connecting wirings provided over both surfaces of the core are necessarily isolated from each other by covering the throughholes with a filling resin. If the throughholes are filled with a conventional prepreg, the heat dissipation is governed by the resin-filled portions due to its low heat conductivity of 0.2 W/mK, which causes a problem in that heat from heat generating components is inefficiently transferred to the core. In addition, when the throughholes are formed by a conventional manufacturing method using the CFRP as the core material, the carbon fiber exposed to the walls of the throughholes is, different from a case of using aluminum, easy to be pulverized into carbon powder, so that the powder is dispersed into the resin in the successive process of filling resin, which causes a problem that reduces electrical insulation quality between the CFRP core and the throughholes or may, in some case, lead to a short circuit therebetween. Moreover, in forming counterbored portions on a metal core substrate for heat dissipation, the pulverized carbon powder is dispersed over the wirings or other substrates in the substrate manufacturing equipment, which causes a short circuit.